Electronic system for x-ray images



July 7, 1959 E. E. sHELDoN l2,894,159

ELECTRONIC SYSTEM FOR X-RAY IMAGES Filed June l, 1951 2 Sheets-Sheet 1 n, mm\ m\\ MMT\&\ U M IIIIN.

E. E. sHELDoN ELECTRONIC SYSTEM FOR x-RAY IMAGES July 7, 1959 2 Sheets-Sheet 2 Filed June l. 1951 QM. NQ)

UnitedStates l ,A to a--m'ethod and` systeml for Aiiitiensifyingl and ,storing irnagefs` of. invisibleradiations and refers more? articular1'y` to`a -rn'ethod and deviceior intensityinga oring `ay images, and Yal'so .,for images vformed by irradiation bybearnsfoatom particles. such as electrons'. or ,.ln'eutrons, and represents acontinuation in part of my..co-pendingfapplicatiom'Sei'. No. 13,916, tiled i i now U.S. Patent No.V 2,555,424, issued lune" i e Y l fe also has subject matter common withrny` U vPatent No. 2,603,7-57.

be-possibleto-reduce .markedly 4the strength `of .radiation affecting Vthe-patients body. Conversely, the exposure time or energy necessary `orexamination, using an invisible radiation. may be considerably reduced. A

.,nothen objective of vthis invention is ,to provide a methodmrandgdeviceto producesharper and. more contrastingi agesof invisible radiations than. it was possible .un-

tilrnewr, e e A; .e other purpose of. thi'spinvention. is...to rprovide the possibility of storing the invisible images andinspecting .them :for Va-. desired length..of..ti`me when wanted without further expenditure.oinvisible radiation, l ,Y

.y Thegpresentintensifying devices concerned with reproduetion.ofX-.ray .fiuoroscopic images werecompletely unsatisfactory, `asin order to obtain improvement in `the visual acuity intensification.offthev `brightness oftheorder i l ot ll.().( 10.is obligatory. YWithout intensiiication of -luminosity of at leastof theorder of 500-100Qthe eye` is coniinedto so-calledscotopic Vision .at which vit is not able to perceive definition and contrast. of `the uoroscopic image. It is well known that. intensification Vof the brightness .of the .Xfray yiiuoroscopic `image cannot be achieved by increase ofX-energyof thegX-frayfrv-adiation, .asitjwill resultindamage .to the patients4 tlssues. Therefore, toobthafin the objectives. of this invention, a special AX-ray sensitive pick-up 4tube had to begdesigned.` This novel vX-ray pick-up. tuberis characterizedrby elimination of the optical system .whichresulted in .20-30 fold .gain in the light reaching thephotocathode. This gain. of incident light on photocathode allowed to activate vthe transmit- -ting system, whichv .before'was notpossible'as with the p amount oi :incident lightl available after passage vthrough the -focusing optical system, Athe signal `to noise ratio was too low `forsatisfactory results.

.To .accomplish the .objectivesof .thisinventiom .a composite X-ray sensitive photoca'thode consisting of an ex- 2,894,159 Patent-.dmv 1. we

. 2' tremely thin photoemissive layer,` a transparent Vlayer-.and an X-ray fluorescent or reactive-layer, is positioned` within novel X-ray sensitive television pick-up tube.` This cornbinationfrepresents a basicimp1'ovement, as it results in 20-3U0 fold gain in light reaching-thephotoemissive-Iayeri The importance of 4this construction is. clear when .it is considered that the most sensitive .television `pick-up turbe` has a threshold'oioperation at above 0.0-1 milli-.lambert which cannot be provided by the energy of X-ray compatible with the patientsfsafet-y.` `It is, thereinej,v obvious that the elimination of the optical systemdisposed between the fluorescent: screen and television system represents an important improvement in securing the. necessary amount of light for operation ofthe X-ray pick-up tube. Still better results-were obtained by theuseof ja very thin light reilecting layer, such asfor aluminum deposited on the surfaceof the iiuorescent layer of the-composite screen, nearest the source of radiation in orderto increase the transfer ofllight to the photoemiss'iyelayer. yIt is also very important to-interposebetween thefiiulorescent.and-photoemissive layers a -very thin light` transparent, chemically inactive separating layer, in. order to prevent vdetrimental'chemical interaction -of said. layers.

The signal to -noise Vratiodeciding the sharpnessand contrast of the imagewas further -improved by the use of a series of composite screensfeach consisting of an electron `transparent light reiiecting layer,y :electron -iiuorescent layer, chemically inactive light transparent layer, and of photoemissive layer; which screensare disposed within the X-ray `pick-up tube inlsucceeding stages. The X-ray image. is converted in'said 'photocathode composite into photoelectron image.. `The photoelectronimage accelerated and v focused by the electrical iields on' the electronpsensitive compositeV screen, whereby an Ain'ttJlsiiied photoelectron -image is produced, which `z,1gair imay fbe focused-on the neXt-electron--sensitive composite screen,

VVproducing Vfurther. 'intensifie-ation -of image. Y

Y. Further intensiiication of the X-ray imag@ was obtained by the use of one or pluralelectron multipliers disposed between the y,composite X-ray `sensitivephotocathodev described `above Vand lthe scanning targetof-the X-raysensitive pick-up tube. The photoeleetron image, Yhaving the pattern of the X-ray image, emitted 'by theV composite X-ray sensitive photocathode, is accelerated and focused by the electric or magnetic elds on the secondary4 electronemissive electrode, whereby an intensified electron image is produced.

In some instances, it is'adVantageoUs to demagnify the electron image emitted by the first composite X-ray-sensitive screen before projecting it on the next composite screen or onV the electron multiplier electrode. The electron diminution of the image results `in its intensification proportional-to the .linear decrease of its size; Next, .the intensified photoelectron image'is stored in the target of the VX-ray sensitive pick-up tube, for a predetermined period of time, then is scanned by electron 4beam and converted into video signals.' Video signals are sent t0 amplifiers. By the use of variable mu amplifiers in one .or .two stages, intensification of video signals can "be produced in non-linear manner, so'that small dierences in intensity of succeeding video signals can be increased one to ten times, producing thereby a corresponding gain of the contrast of the `iifnal visible image in receivers, which was one of the objectives'of this invention.

In some cases video signals are sent to the storage tube having a-storage-target and are deposited there :by modulation of the .electron scanning beam ofv said storage tube. The `stored .electrical charges, having the .pattern of. X-ray images, are. released from-the storage Vtarget after predetermined time by scanning it with another ,electric beam or `by .oodilng it with light. -g `Thereleased electron image is converted again into video signals and sent to final receivers to produce visible image with desired intensification. The invention will appear-more clearly from the following detailed description when taken in connection with the accompanying drawings by way of example only preferred' embodiments of the inventiveu idea.

p In the drawings: Figure 1 shows a sectional diagrammatic view of `a system'for intensifying images of invisible radiations;

Figure 2 shows an improved modification of said system having electron multipliers and a storage tube;

Figure 3 represents a simplified modification of said system;

I liuorescent light and of photoemissive layer 16. Fluovrescent 'substances which may be used' for said com- A Figure 4 represents a modification of said system using optical means therein;

Figure'S represents a'modication 'of said system havinga storage target; Figure 6 is a sectional view of the front portion of the Xray pick-up tube showing an alternate form of the Xray pick-up tube; Figure 7 is a sectional view of the front portion of the Xray pick-up tube showing a modification of the Xray pick-up tube;

- Figure 8 is Ya sectional view ofL the front portion of the Xray pick-up tube showing a modification of the Xray pick-up tube; Figure 9 represents a sectional viewof the front portion of neutron sensitive pick-up tube; Y

Figurev 10 represents a modification of the front portion of neutron sensitive pick-up tube.

Reference will now be made to Figure l', which illustrates new Xray sensitive pick-up tube 1 to-aecomplish the purposes of the invention as outlined above. The X-rays 2 produce invisible Xray image 4 of lthe examined body 3. The invisible Xray image 4 penetrates through the face of the Xray sensitive pick-up tube and activates the composite screen 6 acting as a photocathode, which consists of a very thin Xray transparent light reecting layer 7, such as eg. of aluminum,'of Xray fiuorescent layer 8 with powdered glass', of -chemically inactive light transparent barrier layer 9, and of photoemissive layer 10. The face 5 of the tube is of material transparent to radiation used for examination.

The fiuorescent layer 8 and the photoemissive layer should be correlated, sothat under the influence of the particular radiation used, there is obtained a maximum `photoemissive effect. More particularly, the fluorescent layer should be of a material having its greatest sensitivity to the type of radiation lto be usedand the photoemissive material likewise should have its maximum sensitivity to the wave length emittedA by the fluorescent layer. Fluorescent substances that may be used are: zinc silicates, zinc selenides, zinc sulphide, calcium tungstate or BaPbSO4 with or Without activators. The

satisfactory photoemissive material will be caesium oxide A soV fluorescent layer 8 of the composite photocathode 6 into fluorescent image and then in the photoemissive layer 10 vinto a photoelectron image. The photoelectron image, having the pattern of the Xray image,r is accelerated by electric fields 22 and is focused by means of. magnetic or electrostatic fields 22a on the first comlposite screen17 of the image amplifying section 12 of the tube. The amplifying section 12 has one or a few -suocessively arranged composite screens, each of them consisting of an electron pervious light refiecting layer 13, of layer fiuorescing 14 when irradiated by electrons, i of chemically/inactive .barrier layer 15 transparent to posite screen are zinc silicates, zinc selenides, zinc sulphides, calcium tungstate of BaPbSO4, with or without additional activators; also organic phosphors, such as anthracene or naphthalene are suitable. The satisfactory photoemissive materials are caesium oxide activated by silver, or caesium With` antimony or with bismuth, or antimony with lithium or potassium. V,The barrier layer `15` between the uorescent and the photoemissive surfaces can, be very thin light transparent layer `of mica, glass, ZnF2, of silica, metal or of a suitable plastic. The electron' pervious light reflecting ylayer 13 may be of aluminum or 1ofvsilver'. The photoelectron image from the photocathode 6 focused on the composite screen 17 causes fluorescence of its fluorescent layer 14, which activates the photoemissive layer 16, producing an intensified photoelectron image,k having the pattern of the original Xray image.v The intensified 4photoelectron image can be again 'focused on the nextccomposite screen, whereby its further intensification may be achieved.

In some'v applications, it may bel preferable to use separatelyior in conjunction with the amplifying'section 12, the electron multiplier section 18' consisting of'one or few stagesv of secondary electron multipliers 19, which serves to intensify furtherv the electroninziage.A

VIn such a case,- the photoelectron' image fromj'the composite photocathodev 6, after acceleration is Yfocused by means of electrostatic or magnetic fields 22a on the multiplier. This results in intensification of the electron image by secondary emission from the first stage 19 of the multiplier section. The secondary electrons emitted from the first stage and having the pattern of the Xray image maybe focused after acceleration on the second stage of the multiplier section, producing thereby further intensification of the electron image. The electron image,` produced byelectron multiplier section' of the tube is projected on the'rst'composite screen 17 of amplifying section of the tube for its further'intensitication. The electron image produced vby the amplifying section of the tube is focused on the composite target 21 producing therein pattern of electrical charges corresponding to the Xray image.

In some instances, it is advantageous to demagnify the photoelectron image emittedV by the composite photocathode 6 before projecting it on the composite screen 17. The electron diminution of the image is accomplished by means of electrostatic or magnetic fields which are well known in the art and, therefore, are omitted in order not to complicate the drawings.

The composite storage target 21 lconsists of an electron pervious, light reflecting and therefore light impervious layer 21a, of a layer 2lb fiuorescing when irradiated by electrons,'of chemically inactive layer 21e', 'which is electrically conductive and transparent to' light and of photoconductive' layer,A 21d. Fluorescent substances, which may be used for the composite screen, are zinc silicates, zinc Vselenides, zinc sulphide, calcium tungstate or BaPbSO4, with or without additional activators. The satisfactory photoconductive materials are selenium, CuzO, germanium, thallium sulphide andvlead sulphide or 'selenide. Manysulphides,'selenidesv and oxides exhibit strong photoconduction and can be used for this purpose. vThe layer 21C between the fluorescent and photoconductive surfaces serves as a separating barrier and as a signal plate.- It can be made of a very thin light transparent layer of a conductive'metal or of any other -transparent and conductive material; material kno'wn in trade under name of "Nesa, is very-suitable for this purpose. Y- f The electron pervious light reflectingllayer 21a may be of aluminum or of silver. The electron currentimage from the intensifying screen is focused on the composite target 21 and causes fluorescence of its fluorescent layer alb. which produces .chfulges4 in conductivity in the layer 21nd according to the patternof the original X-r'ay image. The storage target 21 is scanned by elect'ron beam 23 from the electron gun 25. The electron gun is well known in the art and therefore does not need 'any further description. The scanning electron beam f" `may be of a high Velocity such as applied in the icono- Vscope or of a low velocity.

A llow velocity scanning beam is used in the embodiment 'of `the invention shown in Fig. l and it is controlled by the deiiection yoke 66. The scanning beam striking the photoconductive layer 21 deposits electrons thereon fand charges it to the potential of the cathode of the electron gun. The signal plate layer 21e is charged positively from an extraneous source of electrical power.

The resistance of the photoconductive layer is great vrenough to prevent passage of charges from its scanned side to the'positive signal plate. If however the photoconductive layer is illuminated, its resistance decreases proportionally to the intensity of the incident light and fthe: time 'of illumination. This makes possible the ow 'of 'charges through the photoconductive layer and the `scanned side `of said layer becomes between successive scans 11-2 volts positivein relation to the potential of the cathode of the electronv gun. During' the next scan Ythe 'electron beam neutralizes this positive charge on the photoconductive ylayer and produces thereby a video sigrial which flows through the signal plate layer 211.` to `the amplifiers 2S. vThe amplified video signals are sent by "coaxial cable or by high frequency waves to the 'receiver of Ikinescope type 30 or facsimile type in which they are Vrec'onverted into visible images. In a close proximity to thescan'ned side of the photoconductive layer is disposedy a mesh screen 65, which serves to provide a uniform field, in front of the target 21.

Inthe improved form of this invention shown in Fig. 2, the composite target 21 in theX-ray pick-up tube 41 is scanned by a slow electron beam 23a from the electron -gun25a. The scanning electron beam is controlled by the deliection yoke 67 and focusing coil 22h. The scanning 'beam is modulated by the pattern of positive electrical charges on Vthe photoconductive layer 21d. After neutralization of said charges, the returning beam 24 vcarries video information. The returning electron beam strikes the iirst stage of the electron multiplier 26.. The ysecondary electrons from the iirst stage of the multiplier stiike the succeeding stage 27 around and in the back `of the first stage. This process is repeated in a few y stagesresnlting in a marked multiplication of the original "electron signals. The signal `currents from the last stage of the multiplier are fed into television amplifiers and then .sent by coaxial cable or by high `frequency Waves to the receivers o'f kinescope type 30 or facsimile type in which they are reconver'ted into visible images for inspection or for recording. The remaining parts of this pick-up tube 41 are the same 'as described above in Figurel.

In `order to obtain amplification of contrast of the X-ray image, the amplifiers 28 are provided with variable mu `tubes in one or two stages. Small differences in intensity of the succeeding video signals are increased by variable mu tubes in non-'linear manner resulting in again 'ofthe contrast of the visible image in receivers. A marked improvement of the performance of the X-iray Aintensifying system is obtained Yby the use of a storage tube in said system.` Video signals from the -X-.ray sensitive pick-up tube are transmitted to the storage tube and are stored therein. This arrangement allows `shutting'off the X-ray radiation while reading the X-ray image and results in a marked saving of X-ray exposure. Figure 2 illustrates this novel system.

Video signals are `sent from the pick-up tube 41 to thestorage -tube v85 and are deposited there in the form of electric charges,- by means of modulating the scanning i'electron bear'ii 86 of said storage tube, in a special target Y87 in'whioh theyc'an be vst'o'redffcrla predetermined time.

The storage target 87 consists' off a perforated sheet of metal or otherV c`n1ductii`1g-Y material, "or of a woven conducting Wire mesh S7-a. On 'theside ofthe target facing the electr-en gun thereis -dfe'positedV by evaporation storage material 87b in such amanner "that openings in the target should not bevoccludedla In some cases on the side of the target facing vthe electronlgun there is deposited by evaporation. a metal coating. The scanning electron beam 86`isliproduced in the storage tube by the electrongn. 88.and`` is1 modulated Aby incoming video signals from the vX-ray pick-Auptube 41. The scanning electronbeam` may be focusedand deected to produce televisiondikeraster by magnetic orelectrostatic means' which are' Well known the'rart.. This scanning electron beam shouldhave the 'finest spot Icompatible with the requiredintensity of. beam. Between'tle electron gun andthe .storage ina close` spacing tothe target, there is mounted a tine vmesh `conducting.screen 90. On the opposite side of the storage target therexis disposed a metal electrode 89,v which acts as `a collector of the electrons during the reading rphase;V

The -electrons of the scanning beam :strike `the storage target 87 land deposit ythereonfvarying charges-at successive points according to the amplitude'of modulatinginpt signals from the X-ray pick-uptube.' The bestiway of operating my system is .to have the storage surface at zero potential or `atcathode Vpotential and then to write on it positive; it means to deposit positive charges'. This can be accomplished by adjusting the potential ofthe surface of tlhe storage target, so ,that` its secondary emission is vgreater than unity. The secondary Velectrons will be collected by the conducting mesh v87a ofthe storage target or by a collecting electrode and positive charges will `be left on the storage surface. These lpositive -charges deposited on the storing surface ofthe target-may be stored thereon for many hours `depending on .the Atype of the storage material 87bwhich wasused..

`relation to the electron gun cathode. Therefore, vthe electrons of the scanning beam pass through openings 'in the target 87, and are drawn towards electrode 89'.

The'passage of the `scanning electron'beam is modulated by the pattern Vof deposited .charges on the storage target. The `greater is thefpositive charge, .theJmore electrons will pass through the openings in -.the"targ'et. The less positive isthe stored charge, the nfewer? electrons will be transmitted rthrough these openings. 'In this way, the electron beam 86 'scanning .the stora'g'e target in the usual television Yraster will be modulated by the stored image. The transmitted electrons will be collected by the collector 89 and 'will be converted over suitable resistor into video signals. The transmitted electrons may be also multiplied by using, .instead of collector 89', an apertured electrode 'and deflecting fields to make said electrons pass through said aperture 89a in succession and to be fed into multiplier 27a 'before c01iverting them into video signals. a

Video signals havingthe pattern ofthe originalX-ray image, are ampliiied and Atransrriitted by coaxial cable or by high frequency waves to receivers. Receivers may be of various types such as kinescopes, facsimile receivers; electro-graphic cameras and others Amay be used to rcproduce images for inspection or of recording.

After the stored image has been read and no further storage is desired, it may `be erased by the use of the scanning electron beam 86 by `adjusting l.the potential of the storage target tothe value at which the secondary electron emission of its storing surface vis below unity.

.the rellector-89 inthe erasingphase of operation must ybe more negative than of the storage target, so that the .'.scanningelectron beam will be repelled to the storage -targ'et and will neutralize the stored positive charges.

TA simplied Yform of this invention is shown in Figure 3.-..In this-embodiment of this invention, the X-rays 2 ,produce invisible X-ray image4 of theexamined body .3f Theinvisible'X-ray 'image 4 penetrates through the face 5 of the X-ray sensitive pick-up tube 42 and acti- 'yates the composite screen 31 in the X-ray pick-up tube actingas aphotocathode. ,Tlhe screen 31 consists of a very, thinX-ray transparent lightreecting layer `31a, such as of aluminum, of Xray fluorescent layer 31b, of

vchemically inactive light transparent'barrier layer 31e, which is electrically conductive, and of photoconductive layer 31d. The face 5 of the'tubeis of material transparent to' radiation 4and for examination. y :The Vfluorescent'.layerlb is the same as described above. .The light transparent conducting layer 31e may *be'of metal. YThe photoconductive layer 31d may be of selenium, Cu2O,"germanium, thallium sulphide, lead ksulphide or selenide. The 'X-ray image is focused on the -composite photocathodel and causes fluorescence of its .fluorescent layer` 31b, which in turn`produces changes in conductivity inthe layer 31d, according to the pattern of they originalX-ray` image. The photocathode. 31 is scanned by the electron beam 23h from the electron ygun 25b.

The scanning electron beam may be of a high velocity, such as inthe iconoscope orof the low velocity. A low velocity scanning beam is used inthe embodiment of the invention shown in Figure 3 and-is controlledr by the deflection yoke 68 and focusing coil 22e. The scanning beam, striking .the photoconductivel layer 31d, deposits electrons thereonand charges it to thepotential of the Ycathode of the electron gun. The signal plate layer 31e-is charged positively from an extraneous source of -the time of illumination. .This makes possible the flow of charges .through Ythe photoconductive layer and the scanned side of said layer'becomes between .successive scans l--Z volts positive in relation to the potential of the vcathode of the electron gun. During the next scan, the electron lbeam neutralizes the positive charge on the Aphotoconductive layer and produces thereby a video` signal, which `lows through the signal plate layer 31C to the amplifiers. .The amplified video signals are sent b y coaxial cable or Yby high frequency wave to the receivers of kinescope type 30 or facsimile type, in which ,they are reconverted into visible images. In a close proximity to the scanned side of the photoconductive layer, is disposed a mesh screen 65b, which serves to providea uniform ield, in front of the target 31.

It is understood that, instead of a composite photocathode described above (Figure 1), an X-ray reactive layer of electron emissive type may be used as a photocathode. Lead, gold or uranium are suitable for this purpose. 'Y In such case, the electron image from the X-ray reactive layer 29 is focused on lthe amplifying k,screen 17 by means of magnetic or electrostatic elds.

kAn alternative of this invention (see Figure 6) is based on the use of a composite photocathode, in the X-ray pick-up tube 1, having an X-ray reactive layer of electron emitting type 29, such as c g. lead or bismuth, between the face of the tube and light reflecting layer 7b, so that electrons liberated by X-rayradiation from the X-ray reactive layer will excite the adjacent iluorescent layer 8b, whose uorescence will, in turn, activate the v.adjacent suitable photoemissive layer 10b ,through light transparent barrier llayer 9b. In another form o f this invention (see Figure 7), the X-'ray reactive layer 29a of the composite photocathode is in close apposition Vto the secondary electron emissive layer 29h, such as e.g. oiAg or Mg, both layers separated from each other only by very thin electron pervious 'chemically inactive barrier layer 9c.V AIn some instances, it'is preferable to eliminate the barrier layer (see Figure 8) and to focus electron image from the X-ray reactive layer 29a on the electron emissive layer 29b by means of' magnetieor electrostatic elds.

In another alternative of this invention (Figure 4) the X-ray image is converted into a visible fluorescent image in the iluorescent screen A44 positioned outside of the X-ray pick-up tube 45 and is projected by the optical system 46 onto photocathode 47 oftheY X-ray pick-up tube 45. As explained above, the loss ofvlight caused by the use of the optical system makes lit impossible `to activate the most sensitive television pick-up tube by the X-ray uorescent image of the human body. By the use of the. reflective optical system,'there`is obtained 5 to 8 fold gain in light reaching the photocathode of the pick-up tube from the uorescent screen 44. This gain, being still not sucient to activate the standard television pick-uptube, a novel pick-up tube was designed. The .novel pick-up tube 45 is characterized by the amplifying section48, consisting of a single or plural composite screen and of electron multiplying section '49, which both were described indetail above. TheX-ray fluorescent image is projected by the reilective optical system 46, which in this particular case, consists of an aspherical correction plate 46a, of a spherical'concave mirror 46b with an aperture, and of auxiliary plane or convex spherical mirror 46c, onto photocathode 47 /to be converted there into photoelectron image.v

It is to be understood that many various forms o the optical system, such as Maksutovs Schmidtsor an optical system using a meniscus lens, may be used for this purpose and they 4all come within the scope of this in- Y vention. v

The photoelectron image, after multiplication infthe multiplier section 49, and after intensification inv the amplifying section 48 of the pick-up tube, is accelerated by means of electric fields 52 and is focused by means of magnetic or electrostatic fields 57 on the composite target or screen 50, consisting of a light reflecting layer 50a, a uorescent layer Silb, light transparent conducting layer 50c and photoconducting layer 50d. The target is scanned by electron beam 51, which produces videoV signals, as was explained Vabove and illustrated'in Figures l and 2. The electron beam 51 is controlled by deection yoke 55. Video signals are fed into amplier sy'stem 2.8 land therefrom to the immediate or remote receivers to produce visible image with desired luminosity and gain in contrast. Mesh screen 54 provides uniform eld in frontof the target.

The storage of X-ray images, which was another purpose of this invention, may be obtained by means of any of the novel X-ray pick-up tubes, shown in Figures l, 2, 3 and 4. Normally, in the pick-up tube, efforts are made to insure that picture signals are completely erased each scanning cycle, if the fuzzing of moving objects is to be avoided. However, this defect for conventional television, is exactly what is necessary for storage of X-ray images. My storage tube shows vthis time lag, as a result of a very high target capacity in combination with a low velocity and intensity scanning beam. These are the conditions, which cause a long time constant for discharge of a condenser. The composite storage targets `are condensers charged positively by the photoconduction and discharged by the scanning beam current. The target potential builds up proportionately to the time of light exposure. The slope of the target charging curve is determined by the capacity of the target and by the photoconduction. Therefore, the only modifications necessary, in order to use the Xfray pick-up tube the photoconducting layer 50d and conducting layer 50c.

The change in intensity of the scanning electron beam requires only adjustment of the electron gun.

The preferred embodiment of a system for intensifying and storing of X-ray images is shown in Figure 5. The composite photocathode 6 may be the same as shown in Figure 1. An electron image, having the pattern of of the X-ray image, is produced by said photocathode, then is accelerated by electric fields 64 and is focused by the magnetic or electrostatic fields 70 on the composite storage target 60.

The storage target 60 consists of a light reecting layer 60a, fluorescent layer 60b, light transparent conductive layer 60o and a photoconductive layer 60d. The latter layer, in order to have a good storage for charge images, must have a big resistance and must have a minimum thickness compatible with the sharpness of images. Specially prepared for this purpose, ZnS, CdS, CdSe or selenium are suitable for this purpose. The electron gun 62 is designed to produce the scanning electron beam of the lowest intensity compatible with generation of video signals of adequate signal to noise ratio. The elec tron image from the photocathode 6 is converted in the composite storage target 60 in a stored charge image. The scanning electron beam 61 is controlled by the deliection yoke 69. When striking the photoconductive layer 60d, it produces video signals, as was explained above. These video signals have the pattern of the original X-ray image and may be reproduced in receivers, in a visible form for reading. The dissipation of the stored charge image by the action of the scanning electron beam in this construction is a slow process and the image may be read for a few minutes.

The action of the deflecting circuits and of electron gun 62 is of intermittent character. At the time of the X-ray exposure, they are inactive. When the stored X- ray image is -to be read, they are activated, so that the electron beam 61 can scan the photoconductive layer 60d.

After the examination of the X-ray image has been concluded, the composite storage target 60 has to be restored to its original condition, before the next X-ray image can be stored. The photoconductive layer 60d at the end of the reading, has remaining positive charges thereon. In order to neutralize these charges, I spray it with the electron beam from the gun 62 with velocity, at which secondary electron emission ratio of the photoconductive layer is below unity. In this Way, the storage target can be immediately restored to the original condition.

Storage of X-ray images may also be accomplished by using a light feed-back system, as shown in Fig. 1. The liuorescent image in this modication of my invention is reproduced on the face of the kinescope 30a as was explained above, is projected on a television pickup tube 92 and produces a photoelectron image therein. The photoelectron image is converted by the pick-up tube into video signals in the manner well known in television. For the purposes of this invention, any type of television pick-up tube, such as o-f photoemissive type, photoconductive type, or of photovoltaic type may be used. Video signals are sent from the pick-up tube to the kinescope 30a again and reproduce there again the fluorescent image 31. The uorescent image again is projected on the pick-up tube to produce again photoelectron image. In this way, an endless stream of fluo- 10 rescent light images is produced, so that the uorescent image may be inspected for a desired time without maintaining the X-ray exposure. Instead of projecting the fluorescent image from the kinescope on a television. pickup tube92, the uorescent image may also be projected on the photoemissive layer of the X-ray sensitivepickup tube 1 described above, because during the reading of the image,` the X-ray beam is not operating and the X-ray pick-up tube is, therefore, free to be used for regenera- `tion of the examined image.

It is not intended to restrict the scope of this invention to the use of X-ray or gamma rays, but other corpuscular radiation, such as protons or neutrons and suitable reactive layers, should be comprehended. This form-,of the invention is illustrated in Figure 9, wherein a neutron reactive layer 80, preferably from the group boron, lithium, gadolinium, uranium or paratiine is placed within the image tube to act as the neutron sensitive layer of the composite photocathode 6a. The protons or electrons liberated from the layer under the impact of neutron radiation will strike through a thin electron pervious chemically inactive, light reecting layer 7a, a suitable fluorescent layer 8a, causing it to uoresce and activate a suitable photoemissive layer 10a through the light transparent barrier layer 9a. In other cases, a neutron reactive layer of cadmium or copper will be more advantageous, because of gamma emission, which will cause fluorescence of the uorescent layer.

In some cases, it may be more desirable to eliminate the liuorescent layer and to cause protons or electrons from the neutron reactive layer 81 to act on adjacent electron emissive layer 82, such as of beryllium, magnesium or silver, in which case, electron pervious chemically inactive barrier layer may be used to prevent chemical interaction of said adjacent layers. In other cases, better results are achieved by focusing said protons or electrons from the neutron reactive layer 81 on an electron emissive layer 82 with magnetic or electrostatic iields (see Figure 10).

While I have shown a preferred embodiment of my invention, it is to be understood that changes and variations may be resorted to in the elements, combination and construction of my invention, and I reserve my rights to such changes and variations as are within the spirit of this specification, and the scope of the claims hereunto appended.

I claim:

1. A vacuum tube having in combination a screen for receiving an image and converting said image into a current of electrons and a composite target for receiving said electrons, said target comprising luminescent means, a light transparent layer of conducting material and a photoconductive layer having an exposed surface.

2. A vacuum tube having in combination means for receiving an image and converting said image into a current of electrons and a composite target comprising a light reiiecting layer, luminescent means adjacent to said light reflecting layer, light transparent dielectric means, light transparent conducting means and a photoconductive layer.

3. A vacuum tube having in combination a composite screen comprising luminescent means and photoelectric means for receiving an image and converting said image into a current of electrons, and a target comprising a photoconductive layer for receiving said beam of electrons and producing in response to said beam changes of electrical conductivity having the pattern of said electron beam.

4. A vacuum tube having in combination a composite screen comprising luminescent means, and photoelectric means for converting an image into a current of electrons, and a composite target comprising luminescent means, light transparent dielectric means, light transparent conducting means and a photoconductive layer.

5. A vacuum tube having in combination a screen for A* 11 receiving animage and converting said image into a current of electrons, a composite target for receiving said current of electrons, said target comprising luminescentv means, lirght transparent conducting means and a photoconductive layer having one surface exposed, and

means for irradiating said exposed surface of said photoconductive layer with an electron beam.

y 6. A vacuum tube having in combination a screen for converting an image into a current of electrons, a composite target for receiving said current of electrons and converting said current into changes of electrical conductivity, said composite target comprising a 'luminescent means, light transparent dielectric means, a light transparent conducting layer and a photoconductive layer, and

"12 means for irradiating said composite target with an electron beam.

References Cited in the le of this patent UNITED STATES PATENTS Re. 23,802 Sheldon Mar. 16, 1954 2,175,888 Flory Oct. 10, 1939 2,219,113 Ploke Oct. 22, 1940 2,258,294 Lubsznski et al. Oct. 7, 1941 2,344,042 Kailmann et al. Mar. 14, 1944 2,433,941 Weimer Jan. 6, 1948 2,555,424 Sheldon June 5, 1951 2,690,516 Sheldon Sept. 28, 1954 

